Gas turbine engine fuel efficiency typically improves as turbine gas temperatures increase. Consequently, air-cooled superalloy airfoils have been developed to enhance engine performance. Further improvements in turbine performance and component durability can be obtained by the use of protective thermal barrier coatings which insulate the component and inhibit oxidation and hot corrosion (accelerated oxidation by fuel and air impurities such as sulfur and salt) of the superalloy.
A particular type of ceramic coating which is adherent to the metallic component but yet resistant to spalling during thermal cycling, is known as a columnar grained ceramic thermal barrier coating. The ceramic coating layer has a columnar grained microstructure and is bonded to the metal structure. Porosity between the individual columns permits the columnar grained coating to expand and contract without developing stresses sufficient to induce spalling. In accordance with present practice, the metallic article to be protected with the thermal barrier ceramic coating must first be coated with an adherent MCrAlY (M=Ni, Co, Fe) bond coating under layer which is compositionally tailored to grow an adherent, predominately aluminum oxide scale, which inhibits oxidation of the superalloy and provides a satisfactory bonding surface for the ceramic coating layer. The cost of the MCrAlY underlayer, which is normally applied by vapor deposition or other conventional coating techniques, adds substantially to the total cost of the thermal barrier coating system.